Freestyle: control the Thickness of stroked curve using Radius?
Animation Nodes can be used here. Essentially, if you add a 2D curve and copy the 3D spline to it using this simple node tree, you will get what you need:



It also works for multiple splines:



But what if we have multiple spline objects? In this case we append all splines from all objects into a single list and use it instead, and the 3D spline objects can be sourced from a blender object group that the user will add:



What if we want the fill objects(2D Curves) to be separate objects so that we can add different materials for instance. In this case, the 2D curves can be dynamically generated using the Instancer Node: